1. Field of the Invention
The present invention relates to a muffler for an engine.
2. Related Art
A muffler for an engine having: a casing; an exhaust pipe penetrating and extending through the casing; an enlarged chamber having a volume and formed in the exhaust pipe positioned in the casing, the enlarged chamber accommodating therein a catalyst; an upstream side communicating passage communicating an exhaust gas chamber, which is defined between an outer wall surface of the exhaust pipe and an inner wall surface of the casing, and the exhaust pipe upstream of the enlarged chamber with each other; a downstream side communicating passage communicating the exhaust gas chamber and the exhaust pipe downstream of the enlarged chamber with each other, wherein the upstream side communicating passage is closed when the temperature of the exhaust gas is low, and is opened when the temperature of the exhaust gas becomes high, whereby when the temperature of the exhaust gas is low, the exhaust gas flows through the catalyst, and when the temperature of the exhaust gas becomes high, a part of the exhaust gas flows out from the exhaust pipe, via the upstream side communicating passage, into the exhaust gas chamber, and then bypasses the enlarged chamber and returns into the exhaust pipe via the downstream side communicating passage (Refer to Japanese Unexamined Patent Publication No. 9-88568) is known. In other words, in the muffler, in order to prevent the catalyst from being overheated, a part of the exhaust gas is forced to bypass the catalyst when temperature of the exhaust gas is high.
In the muffler above mentioned, when the upstream side communicating passage is opened, a pressure loss of the muffler is decreased. Therefore, a back pressure of the engine can be lowered at this time.
However, in the muffler, the upstream side communicating passage is kept closed as long as the exhaust gas temperature is low. As a result, a problem may occur in which the back pressure of the engine increases when, for example, the engine temperature is low.